The doping of semiconductors with electrically active elements is now performed almost exclusively by ion implanters. Several recent trends in semiconductor technologies suggest characteristics that would be desirable in the design of ion implanters. These trends include the following:
For silicon wafers, the standard wafer size has been increased over the years until 300 mm is the standard diameter used in new facilities today, and manufacturers are planning for 450 mm and larger.
For flat panel displays, dimensions today exceed 550 mm, and larger sizes are to be expected.
As the density of semiconductor chips increases, the implant energy requirements at high doses are decreasing. High doses of boron at energies between 0.1 and 2 KeV will be required of future process tools.
Processes require the ability to control the uniformity of the ion beam density on the semiconductor substrates. Variation across the wafer can cause process failures.
Processes also require the ability to control the angle of incidence of the ion beam on the semiconductor substrates. Angle variation across the wafer can cause process failures.
Processes require the ability to control the energy purity of the ion beam on the semiconductor substrates. Energy contamination can cause process failures as well.
Processes require the ability to prevent the particles coming along with the ion beam density from the upstream beamline from striking the semiconductor substrates. This particle contamination can cause low yield.
A typical ion implanter includes an ion source for generating the ion beam, a beamline system including mass analysis apparatus for mass resolving the ion beam using magnetic fields, as well as a target chamber containing the semiconductor wafer to be implanted by the ion beam. For low energy implantation systems, a deceleration apparatus is sometimes provided between the mass analysis magnet and the target chamber for decelerating the ions to low energies.
In the field of ion beam systems, it is sometimes desired to generate purified ion beams in the form of ribbon-shaped beams. These ribbon beams are commonly used in ion implanter apparatus and implantation systems where a workpiece (such as a silicon wafer or flat panel display) is moved through the ion beam. In these instances, the ribbon ion beam will desirably have a high aspect-ratio such that the beam is wider than any size of the workpiece undergoing implantation; so that in a single pass through the ion beam, a uniform dose of ions may be implanted onto a surface and into the internal substance of the workpiece. In the performance of these applications, it is also very desirable that the ribbon beam has its ion trajectories moving in a parallel direction and be carefully controlled to present a uniform current density profile suitable for the uniform implantation of ions into silicon wafers or flat glass panels.
Among the ion implanter apparatus and implantation systems commercially offered for sale today are those sold for ion beam implantation of flat panel displays (or “FPD's”) by Sumitomo Eaton Nova Corporation (Toyo, Japan), and Ishikawajima-Harima Heavy Industries Co. Ltd. (Tokyo, Japan). These commercially sold systems have, in the past, constituted apparatus and ion beams with little to no ability to reject contaminant species that are almost always present in the beam as it leaves the ion source.
In contrast, Mitsui Engineering and Shipbuilding manufactures implantation apparatus and systems for commercial sale which are able to implant flat panel displays with uniform ribbon beams which have been mass analyzed using magnets that have only modest resolving power (i.e., approximately 2 power)—which is often sufficient to remove the egregious species contaminants from ion beams of several different, commercially useful source elements (See Prior Art FIG. 1, reproduced from U.S. Pat. No. 5,834,786).
Also, Varian Semiconductor Associates Inc. manufactures ion implanters for the implantation of silicon wafers which, in contrast with the aforementioned implanters for flat-panel displays, is an apparatus that uses two different magnets to generate a suitable ribbon-shaped beam. The first magnet mass-analyzes the ion beam; and the second magnet renders the ions in the beam parallel. The resolving power in this Varian two magnet system is as good as any other commercially available ion implanter can provide, and typically exceeds 80 m/dm FWHM. For this reason, this structural format—the two magnetic system—has become the de facto standard for ion beam uniformity and purity and is the system against which all other ribbon-beam implantation systems are currently judged. Unfortunately however, this two-magnet system has severe drawbacks: it is complex and expensive; and is only manufactured to produce ribbon ion beams up to 300 mm in size (See Prior Art FIG. 2, reproduced from U.S. Pat. No. 5,350,926).
Due to the complex interactions between the ion beam and the magnetic field applied for beam expansion, this approach creates severe technical, practical, and process-related problems that increase the total production cost of such equipment and lead to more complicated operation procedures for carrying out the ion implantation. In particular, the beam path through this system is relatively long, and at low energies and high beam currents it becomes increasingly difficult to control the uniformity of the ion beam and the angular variation within the beam to meet the precision required by certain commercial processes.
Nissin Ion Equipment Co. Ltd. (Kyoto, Japan) developed another magnet system to provide ribbon beam for implanting flat panel displays which uses a single large bending magnet, and which achieves a higher magnet resolving power than was available in prior systems. In this Nissin system, the longer (width) dimension of the ribbon-shaped beam is determined by the size of the ion source; and the open spatial gap in the analyzing magnet (from North to South magnetic pole) across which the magnetic field must be developed is larger in size than the dimensions of the ion beam. Structurally, the Nissin analyzer magnet has a steel yoke which is substantially rectangular in configuration and cross-section; wire coils are wound around the sides of this rectangular yoke, more-or-less uniformly; and the coils are subdivided so that the current density may be varied as needed, and thereby control the uniformity of the magnetic field generated within the yoke (See Prior Art FIGS. 3a and 3b respectively, reproduced from U.S. Pat. No. 6,160,262).
Also Advanced Ion Beam Technology (AIBT) has developed another magnet system to produce mass analyzed ribbon beam. An ion source with a slotted aperture produces a divergent ribbon beam. A magnet bends the ribbon beam through an angle between 45 degrees and 110 degrees, and focuses in the direction in which the beam is bent, but does not focus in the direction of the magnetic field. The magnet is similar to the magnet in FIG. 3, but uses so-called bed-stand coil to reduce the fringe field. As a result, the ion beam leaving the magnet is focused to a line focus of high aspect ratio. A lens positioned about the beam near this focus generates a quadrupole magnetic field which provides focusing to cause trajectories diverging in the direction of the long axis of the ion source slot to become approximately parallel. A target may be passed through the parallel beam, thereby implanting across its face a dose of ions with little or no variation in the incident angle of the ions across said surface. (See Prior Art FIG. 4, reproduced from U.S. Pat. No. 7,112,789).
In 2002, Benveniste disclosed a system [See prior art FIG. 6, reproduced from U.S. Pat. No. 6,885,014] which also used two magnets: the first magnet achieved high resolving power in a beam that is a ribbon beam from a ribbon ion source; and the second magnet formed the ions into a parallel beam. The short axis of the beam was aligned with the magnetic field. The significant feature of this system is: it uses a ribbon ion source to provide parallel ribbon ion beam as the input beam for the magnet system.
For ion implanter systems, the system aberration is how identical of the ribbon ion beam seen by the work piece. The no aberration ribbon beam means that each beamlet at different positions of the ribbon beam shall have the same emittance in both the x and the y planes. Any difference in emittance of each beamlet is called an aberration. If the aberration is large, this will cause large variation in the implantation. In the practical world, the magnet design needs to reduce the second order aberration to a certain level. FIG. 5 shows the beamlets at the different positions of a ribbon beam, it also shows the emittance of plot in x-x′ and y-y′ planes of a beamlet.
For example, look at the magnet system in FIG. 2. The first magnet is the mass analyzer magnet which has be designed to reduce the second order aberration. The second magnet is the angle correction magnet which is also designed to reduce the second order aberration. However, the ribbon beam that these two magnets produce still has large aberrations. The fundamental reason is that each beamlet goes through the magnets along different paths where the beamlet's moving velocity vector of V has a different interaction angle with the local magnetic field vector B. According to the Lorentz force, F=qV*B, each beamlet receives different force from the magnetic field. In the end, this brings aberration to the ribbon beam. This aberration can be amplified by the decel system which is one of the reasons that the implanter system of FIG. 2 has difficulty in producing productive low energy ion beams.
Based on the above different mass analyzer magnet, different ion implanter systems have been developed to produce ribbon beams. However, because of the unsatisfactory performance of the mass analyzer magnet, these types of ion implantation systems often are not a viable solution for performing serial mode implantation with a high-current, high-uniformity ion beam that has controllable shapes and sizes. There is a need in the art of integrated circuit fabrication to provide a new system configuration, for generating a high current ion beam that has improved uniformity without requiring additional components in order to reduce the production cost and simplifying the manufacturing processes.